A surge protector or arrester is commonly connected across a comparatively expensive piece of electrical equipment to shunt over-current surges. Such over-current surges occur, for example, when lightning strikes. When this happens, the surge arrester shunts the surge to ground, thereby protecting the piece of electrical equipment and the circuit from damage or destruction.
Present day surge arresters commonly include an elongated, hollow cylindrical housing and a plurality of nonlinear resistive blocks within the housing. Some of these structures also include spark gaps, the blocks and gaps being electrically interconnected to handle voltage and current surge conditions arising on a power line. The blocks commonly contain silicone carbide (SIC) or metal oxide varistors (MOV), and are usually in the shape of relatively short cylinders stacked within the arrester housing. The number of blocks employed is a function of the material (SIC or MOV) and the voltage and current ratings of the assembly.
For a surge arrester to function properly, intimate contact must be maintained between the MCV or SIC blocks. This necessitates placing an axial load on the blocks within the housing. Some prior art arresters utilize bulky contact springs within the housing to provide this axial load. Typically, these springs can provide only relatively small loads, for example, about sixty pounds. As a result, prior art surge arresters experience one or more problems such as poor heat transfer between the MOV or SIC blocks and arrester terminals; non-uniform current distribution, and high contact resistances at joints. Furthermore, units having low contact force sputter and the ionized metal which is produced can cause axial flashover at high currents.
An additional problem with surge arresters of the prior art is that they, on rare occasions, fail in a dangerous fashion. When these arresters fail and experience high fault currents producing high internal gas pressures, the bursting unit may throw parts and cause property damage.
In addition, some of the prior art devices are difficult to assemble, have poor dielectric design, are susceptible to water invasion, and require totally different devices to provide varied voltage ratings.
Examples of prior art surge arresters are disclosed in the following U.S. Pat. No: 2,587,587 to Bellezza et al; U.S. Pat. No. 2,947,903 to Westrom; U.S. Pat. No. 2,997,529 to Fink; U.S. Pat. No. 3,018,406 to Innis; U.S. Pat. No. 3,261,910 to Jacquier; U.S. Pat. No. 3,412,273 to Kennon et al; U.S. Pat. No. 3,524,107 to Reitz; U.S. Pat. No. 3,566,183 to Olsen; U.S. Pat. No. 3,567,541 to Kaczerginski; U.S. Pat. No. 3,586,934 to Nakata; U.S. Pat. No. 3,706,009 to Reitz; U.S. Pat. No. 3,725,745 to Zisa; U.S. Pat. No. 3,850,722 to Kreft; U.S. Pat. No. 3,973,172 to Yost; U.S. Pat. No. 3,987,343 to Cunningham et al; U.S. Pat. No. 4,029,380 to Yonkers; U.S. Pat. No. 4,092,694 to Stetson; U.S. Pat. No. 4,100,588 to Kresge; U.S. Pat. No. 4,107,567 to Cunningham et al; U.S. Pat. No. 4,161,012 to Cunningham; U.S. Pat. 4,218,721 to Stetson; U.S. Pat. No. 4,404,614 to Koch et al; U.S. Pat. No. 4,467,387 to Bergh et al; U.S. Pat. No. 4,491,687 to Kaczerginski et al; and U.S. Defensive Publication T102,103, as well as U.K. patents 730,710; 1,109,151; and 1,505,875.
In the surge arresters of commonly assigned U.S. Pat. No. 4,656,555 to Raudabaugh, the subject matter of which is hereby incorporated by reference, resin soaked glass fibers completely surround and axially compress the varistor blocks. This complete enclosure of the varistor blocks may not permit the gases generated upon varistor block failure to escape to the weathershed housing interior and then out of the weathershed housing before the gas pressure becomes too great and causes the assembly to break apart. If the filament wrap is relatively thin, the wrap can be burned through or can split before an extremely high pressure develops.
Commonly assigned U.S. Pat. No. 5,013,838 to Sakich, the subject matter of which is hereby incorporated by reference, discloses modular electrical assemblies enclosed in an elastomeric housing and having electrical components and end terminals axially compressed by a non-conductive filament winding. The winding is wrapped in a crisscross pattern with lateral openings for venting gas upon failure of one of the electrical components. The metal end terminals are formed by a relatively complex and costly process with radial flanges having notches. The all metallic end terminals developed high electrical stresses in the assembly, particularly in high voltage applications in contaminated environments.
The notched flanges guide the wrapping of the filament winding, and are machined to meet the close tolerances required for proper assembly with the weathershed housing. The machining creates sharp corners, intensifying electrical stresses in the area of the end terminal flange.